The present invention relates to a matrix display apparatus and its driving method; particularly, to a display apparatus and a driving method capable of uniform brightness display, as well as the lowering of signal voltages and power consumption.
One of the important considerations in the design and operation of a liquid crystal display apparatus is the lowering of the driving voltage. Lowering the driving voltage brings about improvement of such factors as picture quality deterioration, non-uniform brightness in the display panel and power consumption reduction. Furthermore, the reliability of circuits in the display apparatus can be improved and a lower price can be realized by downsizing the driving circuits. Especially, in using MOS-LSI techniques in the manufacture of the driving circuit, the price of the display apparatus is lowered significantly because the area of a LSI chip can be made small in size. As mentioned above, it is very advantageous to the picture quality, the power consumption and the price to lower the driving voltage of a liquid crystal display.
Various methods for lowering the driving voltages have been presented. One of them is described in “SOCIETY FOR INFORMATION DISPLAY INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS, (1989), pp 242-244”. The method described in this paper changes the scanning voltage and the counter electrode voltage pulse-wise, in-phase and by the same amplitude, adjusting to the scanning timing for lowering the signal voltage (source voltage) of the voltages driving a TFT (Thin Film Transistor) liquid crystal matrix panel. By this method, the amplitudes of signal voltages can be lowered, but the waveform distortion of the counter electrode voltages and the scanning voltage increases, since parasitic capacitances and resistances of the wiring increase due to an increase in the size of the liquid crystal panel. Therefore, the voltage applied to the liquid crystal changes depending on the pattern displayed on the panel, and a non-uniform brightness and a deterioration of picture quality occurs in the panel thereby. Especially, in a high resolution liquid crystal panel having about a thousand scanning lines, the influences of the waveform distortion becomes severe and the picture quality deterioration occurs significantly due to a short scanning time of one line. Another method for lowering the signal voltage is also described in “SOCIETY FOR INFORMATION DISPLAY INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS, (1992), pp 47-50”. Although it is possible with this method to reduce the deterioration of the picture quality due to the waveform distortion of the scanning voltages and the counter electrode voltages and to lower the driving voltage, direct current voltages are superposed on the voltages applied to the liquid crystal unless the parasitic capacitances between TFT terminals and the storage capacitances are uniform over the panel. Thereby, the phenomena of elongation of a display renewing time, namely, an after-image occurs. The reliability of the liquid crystal decreases by the superposition of the direct current voltage on the liquid crystal. The storage capacitances and the parasitic capacitances have considerable non-uniformity in the panel when the panel size is as much as 10-15 inches, which is as large as the display panel size of personal computers. The non-uniformity is induced because the accuracy of photo-mask adjustment and etching in the process of TFT production degrades proportionately as the panel size becomes large. So the reliability deterioration of the liquid crystal and the of the after-image phenomenon occurrence become more significant as the panel size increases. Other methods for decreasing the signal voltage are presented in Japan Laid Open 913/1990 and Japan Laid Open 145490/1992. These methods do not resolve the problem of poor picture quality, such as the occurrence of a striped picture in the horizontal direction (referred to as smearing).